I think internal routing is only used for really expensive bikes where the owner is willing to spend lots to have what they think is the best looking bike around. And maintenance costs do not matter to people like that.

Or those people that regret getting it on their first bike, never to be gotten again.

Touring, internal routing would create more holes for water to get inside your frame.

If you are bothered by any shadow from your cable when your light is on, move the light or cable. But a good light with a good reflector should not have any shadow from a cable.

My touring bikes have fork crown mounted lights, or no installed headlamp. My heavy duty touring bike, when I am riding near home I put a dyno powered light on the fork crown.



And as you can see above, I have plenty of cables too, including two shift cables on the twist grip shifter on the bar end. And I have plenty of wires for the dynohub power source, the headlamp, a wire to the taillamp, and a wire to the USB charger that is powered by the dynohub.

But when touring on that bike, my light is one that I strap onto the handlebar instead. The light has no battery, only a USB cable, below:



When touring on that bike, the only wire on the bike is from the dynohub to the USB charger. No other wires.

My rando bike, below, has a real spaghetti bowl of wires and cables behind the handlebar bag, below. But I live with it.